Epoxy adhesives have been known for over 50 years and were one of the first high temperature adhesives to become commercialized. Once cured, the material retains its adhesive properties over a large range of temperatures, has high shear strengths, and is resistant to weathering, oil, solvents, and moisture. The adhesive is available commercially as either a 1-part adhesive or 2-part adhesive and is available in several forms, such as pastes, solvent solutions, and supported films. Of the three forms, the 1-part adhesive film generally provides good adhesive strength with better thickness uniformity and has found practical use in the development of anisotropic conducting films for electronics, most notably flat panel displays.
To construct a 1-part adhesive film, one typically combines all at once, a latent hardener, multi-functional epoxy resins, phenoxy resins, additives, and optionally fillers. This composition is then cast as a film on a release layer. During the bonding process, the adhesive is transferred to one particular surface and the release layer removed. Another surface is brought into contact with the film, and the adhesive hardened or cured into a strong thermosetting adhesive through the application of heat and/or pressure. In this example, the two components of the adhesive that enable the material to cure into a thermoset adhesive are the hardener and the multi-functional epoxy. It is the later, that sets up the cross-linked network, but it is the former that enables this to happen. During the curing process, the latent hardener initiates the polymerization of the multi-functional epoxy by first forming ring-opened adducts with the oxiranes of the epoxy resin. Once produced, the addition products cause a cascade of ring-opened species that propagate through the adhesive, finally producing a cross-linked thermoset material.
The active ingredient of the hardener is usually comprised of the reaction product of an amine compound, like an imidazole, and an epoxy resin. Such adducts are known to initiate and accelerate the cure of epoxy resins (Heise, M. S.; Martin, G. C. Macromolecules, 1989, 22 99-104; Heise, M. S.; Martin, G. C. J. Poly. Sci.: Part C: Polym. Lett. 1988, 26, 153-157; Barton, J. M; Shepherd, P. M.; Die Makromolekular Chemie 1975 176, 919-930). One drawback of these however is that they are so effective as curatives they cannot be used directly into a 1-part adhesive because once added, they would start to kick-off the cure in a relative short period of time. What one would see therefore is a slow increase in the viscosity of the composition, while one is attempting to make the adhesive and its film, as the hardener continues to accelerate the ring-opening polymerization of the epoxy moieties. This phenomenon is most commonly referred to as reduced workable lifetime, in other words, the time available to assemble the adhesive and make the film was dramatically reduced because of premature hardening. Therefore, to stop this from happening, one usually does not use amine-epoxy adducts themselves as hardeners, but instead what is typically done is to encapsulate or coat the amine-epoxy adduct with a protective shell of material that sequesters the amine-epoxy adduct from the adhesive environment. Once incorporated into the adhesive, the amine-epoxy adduct is released from its protective shell through the application of heat and/or pressure. Such latent hardeners described here are commonly called to as a core-shell latent hardener, where the core in this case is an amine-epoxy adduct and the shell is the protective shell.
There is one significant trade-off often encountered with core-shell latent hardeners, which is the cure speed is often slowed and the cure temperature often increased because of the inclusion of a protective shell, which must be broken or rendered permeable in order to allow the core material to be released into the adhesive environment or matrix. Without being bound by any particular theory, it is well known that as one increases the barrier properties of the shell material using such means, like increasing the thickness of the shell, cross-linking density, or Tg of the shell, or by increasing the degree of incompatibility between the shell and the core material or the adhesive matrix, it takes more energy to release the amine-epoxy adduct into the adhesive environment. What one has therefore is a hardener that when formulated into a 1-part adhesive has the desired property of increased shelf life stability, but at the expense of a lower curing temperature and a reduction of cure speed. Therefore, it continues to be a constant balance to prepare a core-shell latent hardener that has just enough of a protective shell to protect the core material at normal storage conditions, but not too much as to slow down the cure speed of the adhesive. Also, the release of the core material may be triggered at a reasonably low temperature and completed within a narrow temperature range.
One of the most frequently used core-shell latent hardeners are those comprised of core-shell materials, as described in U.S. Pat. Nos. 4,833,226, 5,219,956, US 2006/0128835, US 2007/0010636, US 2007/0055039, US 2007/0244268, EP 1,557,438, EP 1,731,545, EP 1,852,452, and EP 1,980,580. The hardeners described here are obtained, first by the synthesis of a lump of core material, which is then pulverized into micro-sized particles that are irregular in shape. The core material is the reaction product of an amine compound and an epoxy resin and said core material functions as a hardener for epoxy compositions, such as that found in adhesives and coatings. To improve the storage stability of the core material and prevent premature curing, it is encapsulated with a shell of a material that is impervious to components of the epoxy composition, such as solvent, diluent, low molecular weight epoxides and additives. To accomplish this, the pulverized solid is added to a mixture of polyfunctional isocyanate, an active hydrogen compound, like water, and an epoxy resin. The chemistry of said encapsulation procedure relies on the cross-linking reactions and/or hydrolysis of the polyisocyanate compound to form a cross-linked shell coating around the particles. Typical cross-linking structures of the shell include, but are not limited to, urea, urethane, carbamate, biuret, allophanate, etc. However, the crosslinking reactions take places randomly without discrimination in the continuous phase and at the interface. It is highly likely that some core particles are not fully encapsulated, while unwanted byproducts such as crosslinked polyurea particles are produced in the continuous phase. Moreover, the core particles prepared by this process are of irregular shape with a very broad distribution of shape and particle size, the uniformity of the thickness and crosslinking density of the shell formed thereon is very poor. As a result, the encapsulated hardener particles typically show a very broad distribution of release property and the 1-part adhesive formulated with this type of hardener capsules often shows poor shelf-life stability and a sluggish curing profile or a high curing temperature.
There is another group of inventions, namely EP 459,745, EP 552,976, U.S. Pat. Nos. 5,357,008, 5,480,957, 5,548,058, 5,554,714, 5,561,204, 5,567,792, and 5,591,814, that also describe core shell latent hardeners, which unlike those above are spherical in shape. The core material is obtained as a spherical particle and is synthesized from the reaction of an amine with an active hydrogen atom (e.g., imidazole) and an epoxy resin, in an organic medium and in the presence of a dispersant. The amine, epoxy resin, and dispersant are soluble in the organic medium, while the reaction product, the core material, is not, and as a result the core particle precipitates out from solution as a stable dispersion with a relatively narrow size distribution. The most important factor to make a stable dispersion of desirable particle size with a narrow size distribution is the nature of the dispersant and the inventors show examples that use dispersants from the class of graft of polyacrylates, polyacrylamides, polyvinyl acetates, polyethylene oxides, polystyrenes, and polyvinyl chlorides. Once isolated, the spherical core material is encapsulated with an isocyanate to prepare a spherical core-shell latent hardener.
One disadvantage of the aforementioned latent hardeners is the need of the shell material to be free of defects, such as such as holes, voids, thin areas, or areas comprised of insufficient cross-link density. These defects would enable the core to escape from the protective shell prematurely, either during processing or storage of the finished article. Either way, this premature release of core from the encapsulated latent hardener would show up as a loss of storage stability and shelf-life (in the case of a 1-part epoxy adhesive). This deficiency; however, can be overcome by the application of additional and successive layers of the shell material over the preexisting shell, thus filling in and coating the defects with an additional layers shell material.
Another limitation of the prior art is that in an attempt to make the protective shell more impervious and thereby improving its barrier properties, the compatibility of the shell with the surrounding epoxy composition was neglected. The prior art teaches encapsulation in the presence of an isocyanate, and optionally water and additional epoxy. What one then obtains is a shell comprised of a cross-linked polyurethane and optionally a polyurea. When formulated into an epoxy adhesive, the now hard and highly cross-linked shell could have poor capability with the surrounding epoxy. An example of this would be a mismatch of surface tensions between the surface of the shell and the epoxy; which would show up as a dewetting phenomenon in which the epoxy fails to adequately wet and spread over the surface of the shell material. As a consequence therefore one would see that after curing, the adhesive would contain voids and regions of inhomogeneous curing, both of which would lead to a reduction of adhesive strength.
There remains a need for core-shell latent hardeners with improved barrier properties to prevent premature cure. Additionally, there is a need of encapsulated latent hardeners with improved epoxy compatibility.